1. Field of the Invention
The present invention relates to inductance-change detection apparatus that detects inductance change in coils and more particularly a circuit that compensates inductance characteristics.
2. Description of the Related Art
FIG. 5 is a block circuit diagram that illustrates a prior inductance-change detection apparatus. Referring to FIG. 5, an inductance-change detection circuit 100 detects inductance change in coils L1, L2, where coils L1, L2 are connected in serial. The serial circuit is connected between the plus terminal of a DC power supply 101 and the ground. A resistor 102 is connected in parallel to coil L1. A resistor 103 is connected in parallel to coil L2. These resistors restrict the current flowing through coils L1, L2.
A serial circuit formed of resistors 104, 105 is connected in parallel to the serial circuit of coils L1, L2. The connection between coils L1 and L2 is connected to the inverting input terminal of a differential amplifier 107 through a high-pass filter 106. The connection between resistors 104 and 105 is connected to the non-inverting input terminal of differential amplifier 107 through a high-pass filter 108. The output of the differential amplifier 107 is applied to an inductance-change detector 109. The output of inductance-change detector 109 is the output of inductance-detection apparatus 100. Inductance-change detector 109 detects the inductance change in coils L1, L2 based on the output signal of differential amplifier 107.
Unnecessary low-frequency signals of the coil-voltage-divided voltage VL output from the connection between coils L1 and L2 are attenuated by high-pass filter 106. Then the output of high-pass filter 106 is applied to the inverting input terminal of differential amplifier 107. On the other hand, unnecessary low-frequency signals of a reference voltage V.sub.ref output from the connection between resistors 104 and 105 are attenuated by high-pass filter 108. Then the output of high-pass filter 108 is applied to the non-inverting input of differential amplifier 107. Differential amplifier 107 amplifies and outputs the difference signal between the coil-voltage-divided voltage VL applied to its inverting input and the reference voltage V.sub.ref applied to its non-inverting input.
If, for example, the source voltage VB supplied from DC power supply 101 declines, so that a DC signal is superimposed on the coil-voltage-divided voltage VL and the reference voltage V.sub.ref then the DC signal is blocked by high-pass filters 106 and 108. Therefore, the DC signal does not affect the output of differential amplifier 107. However, the coil-voltage-divided voltage VL has a frequency characteristic, but the reference voltage V.sub.ref does not have a frequency characteristic. Therefore, if an AC signal is superimposed on the source voltage VB, then the output of differential amplifier 107 outputs a signal that indicates as if the inductance of coils L1, L2 has changed. As a result, inductance-change detector 109 wrongly detects inductance change in coils L1, L2.
FIG. 6 illustrates frequency spectra that show the frequency characteristics of coil-voltage-divided voltage VL and the reference voltage V.sub.ref. It is seen from FIG. 6 that the reference voltage does not have a frequency characteristic, but the coil-voltage-divided voltage VL has a frequency characteristic.
Therefore, in order to make the reference voltage V.sub.ref have a frequency characteristic similar to that of the coil-voltage-divided voltage VL, an inductance-change detection apparatus 110 as shown in FIG. 7 has been devised. FIG. 7 differs from FIG. 5 in that a CR circuit 111 is connected in parallel to resistor 105 so that the reference voltage V.sub.ref should have a frequency characteristic similar to that of the coil-voltage-divided voltage VL. By this means, the frequency characteristics of the coil-voltage-divided voltage VL and the reference voltage V.sub.ref become frequency spectra as shown in FIG. 8. As a result, when the detection of inductance change in a low-frequency range is unnecessary, the wrong detection of inductance change in a high-frequency range can be achieved in practice.
However, if the two coils L1 and L2 are placed under different temperature conditions, the frequency characteristic of the reference voltage V.sub.ref does not change, but the frequency characteristic of the coil-voltage-divided voltage VL changes, as illustrated by the frequency spectra of FIG. 9. Therefore, although changes in the coil-voltage-divided VL due to changes in the source voltage VB caused by noise in the power supply can be cancelled out under normal temperature, the coil-voltage-divided voltage VL cannot be cancelled out under low temperature or high temperature. As a result, even if there is no change in the inductance of coils L1, L2, a signal that indicates as if inductance change has occurred is output from differential amplifier 107. Therefore, inductance-change detector 109 wrongly detects inductance changes in coils L1, L2.